Especially MOS-IC's are sensitive to electrostatic discharges. The gate dielectric used therein in existing IC's is so thin that it can break down at a voltage of about 20 to 80 V. However, also bipolar IC's can be damaged, though mostly at higher voltages, from, for example, about 400 V. In the latter case, this is often due to a damage of the base-emitter junction by an abrupt current pulse associated with the discharge. At the area of the pn junction, such a quantity of heat can be dissipated that the semiconductor material recrystallizes locally. This often leads to a permanently weak spot in the pn junction.
In the advancing development of integrated circuits and the technology used therewith, increasingly higher requirements are imposed on the packing density, as a result of which increasingly smaller dimensions are aimed at. As the dimensions in a semiconductor device decrease, the sensitivity to an electrostatic discharge increases. Thus, it becomes more important to provide the connection conductors of the circuit with efficient protection elements, which can adequately counteract the disadvantageous consequences of an electrostatic discharge.
During normal operation, the protection element must not adversely affect the operation of the circuit. This means inter alia that in this case the element must not convey current and that any leakage current must be as small as possible. If, however, an extraordinarily high voltage, exceeding the threshold value abruptly occurs at the connection conductor, the protection element must provide, as soon as possible, for a good conducting current path to the second contact area so that the released charge can be rapidly dissipated along it and it is avoided that the circuit is damaged.
In order to satisfy this double requirement, the protection element may be provided, for example, with a pn junction, which during normal operation of the circuit is biased in the reverse direction. Normally, apart from a small leakage current, no current can flow through the protection element so that the operation of the circuit is not adversely affected. If, however, the voltage across the pn junction exceeds the breakdown voltage thereof, avalanche breakdown can occur. The pn junction then reaches a good conducting state and thus provides for a good conducting current path to the second contact area until the voltage has fallen again.
The breakdown voltage has a given fixed value associated with the pn junction and depends inter alia upon the doping concentrations on either side of the junction. When these concentrations are suitably chosen, the breakdown voltage can be adjusted within certain limits to a suitable value, which then constitutes the threshold value of the protection element. For a satisfactory protection, the pn junction is constructed so that it breaks down before the voltage at the connection conductor has increased to such a value that the circuit can be damaged. Provided that during the breakdown the current density in the protection element has not become too high, the element returns again to its original state afterwards when the voltage has fallen again to a safe level.
The protection element may be constructed as a diode. The pn junction thereof is then used to protect the circuit. Other configurations are also possible. The protection element may, for example, also take the form of a bipolar transistor. The collector-base junction may then be used, for example, to protect the circuit. Another possible configuration is that of a field effect transistor, in which the pn junction between the source or drain and the adjoining part of the semiconductor body may be utilized. In all these cases, the threshold value of the protection element is determined by the breakdown voltage of the pn junction. In general, the choice will mainly be determined by the processing steps available for the manufacture of the remaining part of the semiconductor device.
In an article entitled "Electrical Overstress NMos Silicided Devices" published in "Electrical Overstress/Electrical Discharge Symposium Proceedings 1987, EOS-9, pp. 265-273", a semiconductor device of the kind mentioned in the opening paragraph is described, in which an NMOS transistor is used as the protection element. The transistor comprises an n-type source and an n-type drain, which are both located in a p-type semiconductor body and are mutually separated by a part thereof. In this case, the drain constitutes the active zone mentioned in the opening paragraph. The drain forms with the surrounding part of the semiconductor body the pn junction, which breaks down when the voltage across it exceeds the threshold value. Both the source and the drain of the transistor are coated for the major part with a layer of titanium silicide. The drain is provided with electrodes connected to a connection conductor.
In recent years, use has more or less been made for electrical contacts to semiconductor zones of metal silicides. The zone is then covered with a layer of metal silicide before an electrode, often consisting of aluminium, is provided. In general, the metal silicide can be provided in a self-registered manner, as a result of which the whole exposed part of the zone is provided with the silicide layer without an additional mask being required. This fact as well as the low resistivity, the suitability for use in conventional manufacturing processes and the reliability of the contact between the silicide layer and the subjacent silicon are important advantages offered by metal silicides, such as, for example, titanium silicide.
In the protection elements as described above, the use of metal silicides proves to have an unfavourable influence, however. According to the aforementioned article, NMOS transistors, whose drain is covered with titanium silicide before the electrodes are provided, are 30 to 50% less reliable than comparable transistors, in which the electrodes are contacted directly on the drain by an aluminium contact. A protection element of the first kind is capable, for example, of withstanding considerably lower voltages and current strengths. Such a protection element will thus sooner be destroyed by an electrostatic discharge, which jeopardizes the circuit to be protected.
This problem can of course be solved in that the active zones of the protection element are masked during the deposition of the metal silicide. However, the manufacture of the protection element must deviate as slightly as possible from the process of manufacturing the remaining part of the semiconductor device, in which the use of metal silicides is just desirable and the metal silicides is often provided without a mask. If the metal silicide layer should be omitted solely in the protection element, at least an additional mask would be required.